With the development of the panel display, a-Si and p-Si technologies are usually applied in shift register circuits, and the shift register circuits using these technologies serve as the basic units to form a gate driving circuit of a narrow frame display panel with a high resolution. The oxide thin film transistor (TFT), as a very promising semiconductor technology, is simpler than the p-Si process, has a lower cost, and has a higher mobility than the a-Si, therefore, it has more wide application prospects.
However, the oxide TFT is a depletion transistor, while the a-Si and p-Si mentioned above are both enhancement transistors, the gate source voltage-source current characteristics of the two are as shown in FIG. 1 (enhancement type) and FIG. 2 (depletion type) respectively, the vertical axis thereof is the source current, the horizontal axis thereof is the gate source voltage. From FIG. 1 it can be seen that the source current iD is zero when the gate source voltage vGS is zero, which shows that the enhancement transistor is completely turned off when vGS is 0V. However, as for the depletion transistor in FIG. 2, iD is far greater than zero when vGS is 0V; and iD is zero only when vGS is −6V. That is to say, the depletion transistor is still in the turn-on state when vGS is 0V and cannot be turned off. Therefore, when the shift register circuit that can work normally using a-Si or p-Si in the prior art is fabricated using the depletion transistors, since it cannot be turned off normally, the leakage current is relatively large, the circuit cannot work normally.
Specifically, refer to the conventional shift register unit circuit as shown in FIG. 3. When the circuit is fabricated using enhancement transistors, the circuit can work normally, just as shown in the solid line parts in the work timing diagram of the circuit in FIG. 4. However, if the circuit is fabricated using depletion transistors, since the leakage current is too large, such that the potential of the PU point in the figure cannot be held, and the circuit cannot output normally, the circuit fails, as shown in the dotted line parts in FIG. 4.